Whenever sweeping takes place, the JVM has to make sure the areas filled with unreachable objects can be reused. This can (and eventually will) lead to memory fragmentation which, similarly to disk fragmentation, leads to two problems:

• Write operations become more time-consuming as finding the next free block of sufficient size is no longer a trivial operation.
• When creating new objects, JVM is allocating memory in contiguous blocks. So if fragmentation escalates to a point where no individual free fragment is large enough to accommodate the newly created object, an allocation error occurs.

To avoid such problems, the JVM is making sure the fragmenting does not get out of hand. So instead of just marking and sweeping, a ‘memory defrag’ process also happens during garbage collection. This process relocates all the reachable objects next to each other, eliminating (or reducing) the fragmentation. Here is an illustration of that: